travis: do not allow valgrind failures

Our valgrind jobs haven't been failing for several builds by now.
This indicates that our tests are sufficiently stable when
running under valgrind. As such, any failures reported by
valgrind become interesting to us and shouldn't be ignored when
causing a build to fail.

Remove the valgrind job from the list of allowed failures.

.travis.yml

@@ -47,9 +47,6 @@ matrix:
      os: linux
  allow_failures:
    - env: COVERITY=1
-   - env:
-       - VALGRIND=1
-         OPTIONS="-DBUILD_CLAR=ON -DBUILD_EXAMPLES=OFF -DDEBUG_POOL=ON -DCMAKE_BUILD_TYPE=Debug"
 
 install:
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then ./script/install-deps-${TRAVIS_OS_NAME}.sh; fi